The present invention relates to a waveguide type optical device which is virtually insensitive to temperature change.
With the progress of optical fiber communications, mass productivity, high reliability, dispensing with the adjustments of coupling, automatic assemblage, the lowering of loss, etc. have been required of optical devices, and an optical device of the waveguide type has come into the limelight in order to solve these difficulties.
Among optical waveguides, a silica-based glass optical waveguide is of low loss and also exhibits a slight loss of connection with an optical fiber, so that it is deemed promising as a future optical waveguide. A method of manufacturing the silica based glass optical waveguide has heretofore been a method employing flame hydrolysis deposition as shown in "Recent Optical Waveguide Technology," O plus E, May 1986, p. 63, FIG. 3. It is realized by the formation of a porous silica glass film on a silicon substrate, as well as the formation of a porous silica glass film containing a refractive index-controlling dopant (Ti or Ge), on the first-mentioned film (FIG. 6A of the accompanying drawings), the formation of planar optical waveguide films by annealing the porous films so as to become transparent (FIG. 6B), the formation of a three-dimensional optical waveguide by patterning (FIG. 6C), the formation of a porous silica glass film (cladding material) on the three-dimensional optical waveguide (FIG. 6D), and annealing the cladding material so as to become transparent (FIG. 6E).
With the construction of FIGS. 6A-6E, on a silicon substrate 1, there is formed a buffer layer (SiO.sub.2) 2 which has a coefficient of thermal expansion (5.8.times.10.sup.-7 /.degree.C.) considerably differing from that (4.2.times.10.sup.-6 /.degree.C.) of the silicon substrate, and which is further overlaid with a core layer 7 (a coefficient of thermal expansion: about 4.times.10.sup.-7 /.degree.C.) and a cladding layer 4 (a coefficient of thermal expansion: 5.8.times.10.sup.-7 /.degree.C.). With this construction, however, the coefficients of thermal expansion of the substrate 1 and the glass film 5 formed thereon are too different, so that a residual stress appears within the glass film during the formation thereof. It has been revealed that, for this reason, the following problems are involved: Due to a thermal stress, the so-called index anisotropy that the refractive index of the waveguide in the X-direction thereof and the refractive index thereof in the Y-direction thereof are unequal develops, so light propagation characteristics fluctuate depending upon the direction of an input polarization plane. The warp of the substrate attributed to the difference of the coefficients of thermal expansion appears during the operation of rendering the glass film transparent and after the end thereof, to degrade the light propagation characteristics of the waveguide or to make the packaging thereof difficult. When the glass film thickens due to the residual stress, it cracks and breaks.